1. Field of the Invention
The present invention relates to a planar illumination device including: a light guiding plate; a light source arranged at a side surface of the light guiding plate; and a frame for accommodating therein these components.
2. Description of the Related at
At present, liquid crystal display devices (LCDs) are used as display devices of electronic devices, such as personal computers and mobile phones, and since liquid crystal is not a light emitting type display element, planar illumination devices are widely used as illuminating means used in combination with liquid crystal display devices.
A schematic configuration of a planar illumination device 100 includes, as illustrated in FIGS. 6A-6C, components, which are: a light guiding plate 12, which is formed of a synthetic resin material, has one (surface) of a pair of opposite principal surfaces serving as a light emission surface, and is rectangular in a plan view thereof; a reflecting sheet 118 arranged on a reverse surface side of the light guiding plate 12; and plural plate shaped (or sheet shaped) members, such as an optical sheet 18 of plural layers (three layers in the example in FIGS. 6A-6C) arranged on a surface side of the light guiding plate 12. Inside a frame 116, which, in the example of FIGS. 6A-6C, has a bottomed box shape and is made of sheet metal, these components are stored, together with a light source 14, such as a white LED, which is arranged opposite to a side surface (a light incident surface) of the light guiding plate 12, the side surface connecting end edges of the pair of principal surfaces.
The frame 116 made of sheet metal includes a bottom portion 116Ma, a first side wall 116Mb, and a second side wall 116Mc. The first side wall 116Mb is arranged along each of a pair of side surfaces 12f connecting end edges of a side surface of the light guiding plate 12 at a light incident surface 12d side with end edges of a side surface 12e opposite to the light incident surface 12c. The second side wall 116Mc is arranged along each of the light incident surface 12c of the light guiding plate 12 and the side surface 12e opposite to the light incident surface 12c. The bottom portion 116Ma is arranged to block a region surrounded by the first side wall 116Mb and the second side wall 116Mc. Both the first side wall 116Mb and the second side wall 116Mc are configured to stand up integrally with and from the bottom portion 116Ma.
Furthermore, in the frame 116 made of sheet metal, a pair of resin portions 116Ra and 116Rb are arranged, which are respectively opposite to the light incident surface 12c of the light guiding plate 12 and the side surface 12e opposite to the light incident surface 12c. The resin portions 116Ra and 116Rb are, for example, bar shaped members, having rectangular cross sections. In the example of FIGS. 6A-6C, the resin portions 116Ra and 116Rb are integrated with the frame 116 made of sheet metal, through insert molding (for example, see Japanese Patent Application Laid-open No 2004-240239).
Moreover, a light shielding sheet 20 prescribing an effective area of the emission surface is used as a means to fix the optical sheet 18 to the frame 116 and the planar illumination device 100 to an LCD (illustration thereof omitted). In the example of FIGS. 6A-6C, the light shielding sheet 20 is a double sided adhesive sheet, and one of surfaces thereof (lower surface) is stuck over an upper surface of the optical sheet 18 and an upper surface of the frame 116. The other one of the surfaces (upper surface) is stuck to the LCD (see the specification of Japanese Patent Applioation No. 2014-216444).
One of reasons for using the frame 116 made of sheet metal like in the example of FIGS. 6-6C is to meet a demand for frame narrowing of the display device and the planar illumination device, which is the illuminating means of the display device, in order to downsize the liquid crystal display device and to improve the design. In order to deal with reduction in adhesive area of the light shielding sheet 20 with respect to the upper surface of the frame 116 made of metal, the reduction in adhesive area caused by the frame thickness reduction, the pair of resin portions 116Ra and 116Rb, which are arranged at positions respectively opposite to the light incident surface 12c and the side surface 12e opposite to the light incident surface 12c, are also used as a means to secure the adhesive area of the light shielding sheet 20.
However, for the planar illumination device, in addition to the reduction in thickness and securing the necessary rigidity, further frame narrowing is demanded.
The present invention has been made in view of the above described problems, and an object thereof is to balance characteristics with one another even more highly, the characteristics such as frame narrowing, reduction in thickness, and securing necessary rigidity, which are demanded for a planar illumination device.